The present invention relates to a toner for developing latent electrostatic images in electrophotography, electrostatic recording, electrostatic printing and the like.
In electrophotography, a latent electrostatic image is formed on a photoconductor by a variety of conventional methods and the latent electrostatic image is then developed by use of a toner to a visible toner image. When necessary, the developed toner image is then transferred to a transfer sheet and fixed thereto, for instance, by application of heat thereto.
As the method of developing a latent electrostatic image to a visible image, a variety of methods, such as magnetic brush development method, cascade development method and powder-cloud method, are known. In any of these development methods, the step of fixing toner images to a transfer sheet is particularly important.
Currently as the method of fixing such toner images to a transfer sheet, methods of applying heat to the toner images to soften or fuse a thermoplastic resin contained in the toner particles and fix the toner particles to the transfer sheet are in general use, for instance, by using a heat roller for fixing toner images, or fixing toner images in a heated atmosphere of by a flash light. Of these methods, the image fixing method using a heat roller (hereinafter referred to as the heat roller image fixing method) is used most at present because of its quick energization and high heat application efficiency with a minimum thermal energy consumption.
However, in this heat roller image fixing method, a heat roller comes into direct contact with a toner which is fused or melted by application of heat thereto. Therefore, part of the melted toner adheres to the surface of the heat roller and the toner adhering to the heat roller is then successively transferred to transfer sheets, thus the so-called offset phenomenon takes place.
As a method of preventing such an offset phenomenon, there is known a method of applying silicone oil to the surface of a heat roller to minimize the adhesion of the toner to the heat roller. In this method, since the step of applying an oily material such as silicone oil to the heat roller is necessary, an image fixing apparatus for this method is complex in mechanism.
Another method of preventing such an offset phenomenon, a method of providing a toner itself with an offset preventing property has been proposed, in which a binder resin having a high glass transition temperature (Tg) and a high molecular weight is employed in the toner to elevate the melting viscoelasticity of the toner. This method, however, has the drawbacks that the image fixing performance of the toner is insufficient for use in practice and the toner cannot be used for image fixing at low temperatures for attaining high speed image fixing and saving thermal energy.
In general, in order to improve the low temperature image fixing performance of a toner, it is necessary to decrease the viscosity of the toner when melted and to increase the contact area thereof with a transfer sheet. In order to attain this, it is required that the glass transition temperature (Tg) and the molecular weight of a binder resin contained in the toner be decreased.
Generally since the low temperature image fixing performance and the offset preventing performance contradict each other, it is extremely difficult to develop a toner having such contradictory properties.
In accordance with the recent quick development of stencil printing and the recent demand for polyvinyl chloride products for stencil printing, a variety of requests have been made with respect to the copied materials using toner.
For instance, in the conventional stencil printing, that is, mimeograph, a stencil master paper is manually cut, for instance, with a steel pen. Recently, however, a printing plate using the stencil paper is quickly made by the following method. A stencil master paper is superimposed on an original which bears images. A thin film made of, for instance, polyester, is further superimposed on the master paper. This composition is exposed to flash light, so that the portions of the thin film corresponding to the images of the original are caused to fuse instantly and perforated in such a degree as to be proportional to the image densities of the images, by utilizing the differences in the light absorption degree of the film, thus a printing plate is instantly made.
In such a plate making method using a stencil paper, when an original which bears toner images is employed, the toner of the fixed images is fused in the course of the above plate printing process and adheres to the perforated portions of the film. When this occurs during the printing plate making process, the image quality obtained by the printing plate is significantly decreased.
For the purpose of solving this problem, for instance, in Japanese Patent Publication 51-23354, there is proposed a toner comprising a vinyl polymer which is appropriately cross-linked by use of a cross linking agent and a molecular weight adjusting agent, and in Japanese Patent Publication 55-6805, there is proposed a toner comprising a polymer with a repetition unit of an .alpha.,.beta.-unsaturated ethylene monomer, with the molecular weight distribution thereof being broadened in such a manner that the ratio of the weight average molecular weight to the number average molecular weight falls in the range of 3.5 to 40. Further, there has been proposed a toner comprising blended vinyl polymers with different glass transition temperatures (Tg), molecular weights and gel contents.
Unquestionably, in the above-mentioned toners, the temperature range between the lower limit temperature for image fixing and the offset initiation temperature is broadened. However, when the toners are formulated so as to have a sufficient offset preventing property, the image fixing temperature cannot be sufficiently lowered. On the other hand, when the image fixing temperature is sufficiently lowered, a sufficient offset preventing property cannot be obtained.
In Japanese Laid-Open Patent Application 57-208559, there is proposed a toner comprising a cross-linked polyester resin and an offset preventing agent. This toner is excellent in the low temperature image fixing performance and the offset preventing performance. However, the toner has the shortcoming that it adheres to a heated stencil master paper. This shortcoming cannot be eliminated even if a variety of die lubricants serving as offset preventing agent are added to the toner.
Further, in Japanese Laid-Open Patent Application 54-114245, there is proposed a toner comprising 50 to 95 wt. % of a polyester resin having a low molecular weight and a low melting point and a vinyl resin having a weight average molecular weight of 500,000 or more, which is excellent in both the low temperature image fixing performance and the offset preventing performance. In this toner, the difference in melting viscosity between the two resins is so large that the toner cannot be obtained in a good dispersing state even if the components are fused and kneaded for a long period of time with application of strong shearing force thereto.
Generally it is reported that when uncompatible resins are blended, these resins form an island-sea structure. This theory is described, for instance, in Plastics 13, No. 9, p. 1 (1962). Improvement of the dispersibility of a resin component means to reduce the size of the islands of the resin component. For example, when a plurality of resins is fused and blended, the lower the melting viscosity of each resin, the better the dispersed state of the blended resins.
An observation of the dispersing state of a toner shown in Example 1 of the previously mentioned Japanese Laid-Open Patent Application 54-114245 by use of a transmitting-type microscope indicates that a styrene-butadiene resin is present in the form of large islands in a sea-like polyester, with non-uniform dispersion of carbon black, and that the toner particles are not uniformly dispersed. When there toner particles are mixed with an iron powder carrier, the charge quantity distribution of the toner is abnormally broad. When this toner is used in repetition for making copies, the fogging of images takes place. When a stencil printing master is prepared by use of an original which bears images formed by fixing this toner, a large amount of the toner adheres to the stencil printing master, and the images printed by the master are not clear.
Japanese Laid-Open Patent Application 59-107359 discloses a toner comprising 5 to 45 wt. % of a condensed polymer prepared by polymerizing a polyfunctional monomer, 20 to 60 wt. % of which polyfunctional monomer is a three or more functional monomer, and 95 to 55 wt. % of a polymer prepared by polymerizing a vinyl monomer. This monomer has the shortcoming that the low temperature image fixing performance is insufficient because of the high viscoelasticity of the condensed polymer which has a three-dimensional network structure.
In addition to the above, there has been proposed a method of using in a toner a graft copolymer consisting of a polyester resin and a vinyl resin with a common segment introduced into the two resins in order to improve the mutual dispersion of the two resins. This toner, however, is not sufficient in the offset preventing performance and low temperature image fixing performance.
The contamination of polyvinyl chloride products with a conventional toner is caused as follows. A plasticizer such as dioctyl phthalate (DOP) and dibutyl phthalate (DBP) contained in a soft polyvinyl chloride is transferred to the toner, the viscosity of the toner is decreased by the plasticizer, so that the toner adheres to the polyvinyl chloride products.
Conventionally, as a binder resin for a toner for developing latent electrostatic images, a styrene copolymer and a styrene - acrylate or methacrylate copolymer are in general use. These polymeric resins are compatible with the plasticizers contained in polyvinyl chloride. Therefore the contamination of soft polyvinyl chloride products with the toner is apt to occur under the above-mentioned process.
Several methods of preventing such contamination of polyvinyl chloride products with the toner have been proposed. None of them, however, can attain the low temperature image fixing performance which is one of the most important requirements for the toner and the prevention of the adhesion of the fused toner to the master paper for stencil printing, although the contamination of vinyl chloride products with the toner can be prevented to some extent.
For instance, Japanese Patent Publication 55-10906 and Japanese Laid-Open Patent Application 59-166965 propose a method of introducing a hydrophilic group into a binder resin. The toner using such a binder resin is capable of preventing the contamination of polyvinyl chloride products to some extent, but has a fatal drawback in that the charge quantity of the toner significantly changes depending upon the ambient conditions such as temperature and humidity, and accordingly the image quality obtained by the toner changes depending upon such ambient conditions.
Further, Japanese Laid-Open Patent Application 59-162564 proposes toners in which a homopolymer or copolymer of acrylate or methacrylate, having an alkyl group having 3 or less carbon atoms, or a copolymer of styrene and acrylate or methacrylate with the styrene monomer unit thereof being 30 wt. % or less, is employed as a binder resin. In both cases where the homopolymer of acrylate is used and where the copolymer of styrene and acrylate or methacrylate, with the content of styrene being 30 wt. % or less, is used, the glass transition temperature (Tg) of the polymers is so low that the toners containing such binder resins tend to aggregate during storage. When the homopolymer or copolymer of methacrylate is employed, the glass transition temperature (Tg) is so high that the preservability of the toner is good, but the image fixing performance is insufficient for use in practice.